OSLO/BEIJING (Reuters) - When the International Energy Agency reported in March that global carbon emissions had stayed flat in 2014, even as the world economy grew, the news was hailed as a turning point in the struggle to curb climate change.

But more recent data about Chinese coal consumption, seen by Reuters, raise doubts about whether that historic decoupling of economic growth and carbon emissions from energy use actually occurred.

One of the keys to keeping carbon emissions flat in 2014 was significantly lower coal consumption in China, the world's top greenhouse gas emitter: a 2.9 per cent drop, reported in preliminary Chinese data in February.

It was the first fall in coal use by China this century. And it was good news for the U.N. climate conference meeting in Paris in December with the aim of stopping temperatures rising more than 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial levels: the limit beyond which scientists say the world will suffer ever-worsening floods, droughts, storms and rising seas.But in May, China’s National Bureau of Statistics (NBS) released a China Statistical Abstract, not available online but only on paper, showing that coal consumption edged up by 0.06 percent from 2013.

Just that difference between two sets of NBS data would in turn lift global emissions growth in 2014 from a flat line to about 0.5 per cent, in line with an estimate by oil company BP in a report in June.

That global growth rate is still low, but would undermine the arguments of many, from environmental groups to governments, who have cited the IEA data to support the idea that cuts in carbon use need not necessarily hamper economic growth.

When the IEA report first came out, the agency issued a statement trumpeting that carbon emissions from energy had stalled in 2014, marking “the first time in 40 years in which there was a halt or reduction in emissions of the greenhouse gas that was not tied to an economic downturn.”

CARBON PUZZLE

Until now, many foreign researchers have been unsure which set of Chinese data to trust. Both put 2014 coal consumption at 2.81 billion tonnes, but each infers a different change from 2013.

Jiang Kejun, a senior researcher with the Energy Research Institute, a think-tank under the National Development and Reform Commission, told Reuters both were correct.

He said the 2.9 percent decline referred to the quantity of all coal used, but the 0.06 percent rise in the Abstract referred to the energy released by burning coal, expressed in standard tonnes.

Since China does not publish its annual greenhouse gas emissions, it is still hard to know how to convert coal use to carbon dioxide (CO2) release, but Glen Peters of the Center for International Climate and Environmental Research in Oslo said the Abstract's number was a better guide.

The coal burnt in 2014 was of higher quality because China introduced new rules late last year banning the sale, transportation, import and combustion of low-grade coal. And a tonne of high-quality anthracite coal releases much more carbon when burnt than a tonne of low-grade lignite.

"There's a lot riding on one single number," Peters said. "It’s the difference between being able to say ’We’re getting on track for 2 degrees’ or not."

BIG REVISION?

A report last month in the journal Nature did say international organizations may be overestimating China's emissions, partly because China's coal is lower quality than assumed and so releases less carbon when burnt.

The IEA says it is now awaiting Chinese data from an energy census and other revisions before issuing any update.

"We are aware that there is the potential for a big revision, not only for the 2013/2014 growth rate but also for a longer time series," IEA spokesman Greg Frost said.

BP used the Abstract's numbers in a report last June that found world emissions rose 0.5 percent in 2014, according to BP spokesman Toby Odone.

That would still be the lowest rise in any year since 1998, other than the year following the 2008 financial crisis.

But Reuters calculations show that, using China’s figures from February, BP would have concluded world emissions dipped 0.1 percent between 2013 and 2014.

This year, a broad economic slowdown in China meant that coal production fell 4.8 percent in the first eight months, suggesting that the slowing trend may continue for a time, at least.

Consumption data are not yet available but Li Shuo of Greenpeace, citing data on output and imports, said there was "zero doubt that China's coal use has been falling for at least 12 months."

Yet while CO2 from burning fossil fuels accounts for the biggest single share of greenhouse gas emissions, other sources include carbon from cement production and emissions of more potent gases such as methane or nitrous oxide - most of which are rising.

And even a fall in CO2 emissions would take years to manifest itself in the atmosphere.

"Emissions have been at a record rate in the last few years, and so has the rate of carbon dioxide increase," said Pieter Tans of the U.S. National Oceanic and Atmospheric Administration, which measures CO2 concentrations on a mountaintop at Mauna Loa, Hawaii.

(Reporting By Alister Doyle, David Stanway and Kathy Chen; editing by Bruce Wallace and Kevin Liffey)